1. Technical Field
This invention relates to power supply circuits for computers or the like, and more particularly to power supply circuits using two or more separate redundant regulators (or other supplies such as batteries) to supply a common load, and having a means for isolating (shutting down) each of the regulators during a diagnostic mode of operation.
2. Description of the Related Art
Large computer systems are often provided with a back-up power source, so that in case of a failure of the main power source (usually AC line current), the computer can be operated long enough to save its state. When the power comes back up, the computer can be restarted without loss of data. During the time that the computer is down, it is common practice to maintain some parts of the system in a standby, power-down mode, so that contents of main memory, for example, are kept intact. To supply this standby power, usually batteries are employed, and in a large system the battery requirements are substantial. The main memory may be maintained at a current which is lower than the ordinary operating current, further reducing power requirements during this period. For example, DRAMs used for the main memory may be placed in a mode of lengthened clock and refresh periods. It is also desirable to have converters so that if one goes out the standby system is not totally defeated. When going into the standby mode, i.e., just after main AC power goes down, it is preferable to do a diagnostic routine to make sure the separate converters are operating properly, and if not then provision is made for individually shutting off the converters to prevent a faulty converter from disturbing the load. For this reason, the converter outputs are isolated from one another by diodes. Unfortunately, the diodes are still in the circuit after the diagnostic routine is completed, and the forward voltage drop represents a substantial power dissipation which is of course adverse to long battery life. At the low voltages involved, the forward drop of these diodes is a significant fraction of the total power usage in the battery-supplied standby or backup mode.